An effective ex-situ method for characterizing electrochemical durability of a gas diffusion layer (GDL) under simulated polymer electrolyte membrane fuel cell (PEMFC) conditions is reported in this article. Electrochemical oxidation of the GDLs are studied following potentiostatic treatments up to 96 h holding at potentials from 1.0 to 1.4 V (vs.SCE) in 0.5 mol L(-1) H(2)SO(4). From the analysis of morphology, resistance, gas permeability and contact angle, the characteristics of the fresh GDL and the oxidized GDLs are compared. It is found that the maximum power densities of the fuel cells with the oxidized GDLs hold at 1.2 and 1.4 V (vs.SCE) for 96 h decreased 178 and 486 mW cm(-2), respectively. The electrochemical impedance spectra measured at 1500 mA cm(-2) are also presented and they reveal that the ohmic resistance, charge-transfer and mass-transfer resistances of the fuel cell changed significantly due to corrosion at high potential. (c) 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.